As an image forming apparatus capable of recording an image on a substrate (hereinafter called a recording medium) having a low ink absorbability such as resin film besides ordinary substrates such as a paper and a texture, there has been developed an inkjet recording apparatus which lands ink ejected from a nozzle disposed at an end surface (a so-called nozzle surface) of a recording head on the substrate, and at the present day, a technology of the apparatus is applied to various technical fields.
In so doing, as the recording head used in the inkjet recording apparatus, a recording head in which nozzles are arranged in rows in parallel on the nozzle surface is often used However, in recent years, as FIG. 24 shows, there has been often used a recording head having three nozzles rows 3m, 3m+1 and 3m+2 in which a plurality of nozzles N are arranged in a staggered form with a predetermined interval in a main scanning direction X indicated by an arrow X in the figure (for example, refer to Patent Document 1). Incidentally, in FIG. 24, under the recording head H, namely on a back side in the figure, there is a recording medium S, and nozzles are formed on a nozzle surface P, wherein the nozzle surface P represents a side, which is facing the recording medium S, of the recording head.
As above, the arrangement of the nozzles N in which nozzle positions of the adjacent nozzles N are displaced in the main scanning direction X is called a staggered arrangement. The staggered arrangement is often configured by repeating an arrangement to displace the nozzle positions for every three or four nozzles N. In case the nozzle positions are displaced every three nozzles N, generally, each nozzle N3m denoted by N3m (m=0, 1, 2 . . . ) is arranged in an row in a sub-scanning direction perpendicular to the main scanning direction, and the nozzles N3m+1, and N3m+2 denoted by N3m+1 and Nm+2 are arranged in an row in the sub-scanning direction Y respectively.
Supposing that a pixel pitch on the recording medium S is L, a nozzle interval p between each of rows of nozzles N3m, N3m+1 and N3m+2 is configured to be ⅓ or ⅔ of the pixel pitch L, namely L/3 or 2L/3. Also, an interval q between each adjacent nozzle N in the sub-scanning direction Y is usually configured to be equal to the pixel pitch L.
Incidentally, each nozzle N3m belongs to the same row is collectively called nozzle row 3m. Namely, a nozzle row configured with each nozzle N3m denoted by the nozzle number 3m (m=0, 1, 2, . . . ) is called a nozzle row 3m, and the a nozzle row configured with each nozzle N3m+1 denoted by the nozzle number 3m+1 is called a nozzle row 3m+1 and the a nozzle row configured with each nozzle N3m+2 denoted by the nozzle number 3m+2 is called a nozzle row 3m+2.
The nozzles N in the staggered arrangement are usually driven by a multi-phase and in the recording head H in which the nozzles N are disposed in the staggered arrangement by three nozzles as FIG. 24 shows, each of nozzles N3m, N3m+1 and N3m+2 is driven by three-phase. The three-phase drive of each of nozzles N3m, N3m+1, N m+2 having been conducted conventionally is conducted as follow.
Namely, as FIG. 25 shows, first, to change the phase of ejection drive, a strobe pulse STB1 is applied to each of the nozzles N3m in the nozzle row 3m, and in this state by applying an unillustrated drive pulse, ink is ejected to the recording medium S from each of the nozzles N3m in the nozzle row 3m to which the strobe pulse STB1 is applied.
Then, at the time when the recording head H is moved by ⅓ of the pixel pitch L in the main scanning direction X, the strobe pulse is changed from STB1 to STB2 and applied to each of nozzles N3m+1 in the nozzle row 3m+1. In this state, by applying the drive pulse, the ink is ejected to the recording medium S from each of the nozzles N3m+1 in the nozzle row 3m+1 to which the strobe pulse STB2 is applied. When this occurs, since the nozzle N3m+1 is behind the nozzle N3m by L/3 in the moving direction of the recording head H in the main scanning direction X, the ink ejected from each of nozzles N3m+1 lands on an adjacent position to the ink ejected from each of nozzles N3m in the sub-scanning direction Y on the recording medium S.
In the same manner, at the time when the recording head H is further moved by ⅓ of the pixel pitch L in the main scanning direction X, the strobe pulse is changed from STB2 to STB3 and applied to each of nozzles N3m+2 in the nozzle row 3m+2. In this state, by applying the drive pulse, ink is ejected to the recording medium S from each of the nozzles N3m+2 in the nozzle row 3m+2. On the recording medium S, the ink ejected from each of nozzles N3m+2 lands on an adjacent position in the sub-scanning direction Y on which the ink ejected from each of nozzles N3m+1 has been landed.
As above, by changing the strobe pulse from STB1, STB2 to STB3 sequentially so as to change driving phases, the ink ejected from each of nozzles N3m to N3m+2 can be landed on a line which extends in the sub-scanning direction on the recording medium S. Meanwhile, as above, a method to change the phases to drive from the nozzle N3m in a front section to the nozzle N3m+1 on a rear side sequentially in the moving direction of the recording head H in the main scanning direction X is called normal phase.
Also, by further moving the recording head H by ⅓ of the pixel pitch L in the main scanning direction, each of the nozzles N3m in the nozzle row 3m comes to a position which is distant the pixel pitch L from the position where the ink is first ejected. At that time, by applying the drive pulse in a state where the strobe pulse is changed from STB3 to STB1 and is applied to each of the nozzles N3m in the nozzle row 3m, the ink is ejected from each of the nozzles N3m in the nozzle row 3m to a position displaced by the pixel pitch L in the main scanning direction X from a position where the ink has been first ejected. Therefore, the ink lands on an adjacent position to the position when the ink has been first ejected from each nozzle N3m in the nozzle row 3m. 
As above, every time the recording head is moved by L/3 in the main scanning direction X, by changing the strobe pulse from STB1 to STB2 and to STB3 sequentially, the ink is landed on the line extending in the sub-scanning direction, and the ink is also landed on another line in an adjacent position to the above line of the ink extending in the main scanning direction X on the recording medium S. In a conventional inkjet recording apparatus, by repeating operation to land the ink in the lines extending in the sub-scanning direction Y on the recording medium S, the ink lands on each pixel on the recording medium S so that an image is recorded on the recording medium S.
Meanwhile, as FIGS. 24 and 25 show, in case the recording head H moves to a left side in the figure, by changing the strobe pulse from STB1 to STB2 and to STB3 sequentially, the nozzle N to eject ink is changed from the nozzle N3m in the front section in the moving direction of the recording head H to the nozzle N3m+1 and to the nozzle N3m+2 on the rear side, however in case the image is recorded while the recording head H is being moved to the left side in the figure, the nozzle N in the front section in the moving direction of the recording head H is changed to the nozzle N3m+2, thus the strobe pulse is changed from STB3 to STB2 and to STB1 sequentially so that the nozzle N to eject ink is changed from the nozzle N3m+2 in the front side in the moving direction of the recording head H to the nozzle N3m+1 and to N3m on the rear side.
However, as above, when the image is recorded on the recording medium S, for example, if the ink is not ejected from the nozzle N normally because a specific nozzle is defective, as denoted by x, a portion to which ink is not ejected in a line shape extending in the main scanning direction X is formed in a corresponding position, thus a streak-like pattern appears in the image recorded on the recording medium S which deteriorates image quality. Incidentally, as FIG. 25 shows, if the nozzles N disable to eject ink are described in the figure, the figure becomes complicated, thus only positions where the ink is landed are denoted by circles in the figures below including FIG. 26.
In order to solve the above problem, in an inkjet recording apparatus in Patent Document 2, for example, as FIGS. 24 and 25 show, a so-called reverse phase is suggested that is when the recording head H is moved to a right side in the figure, instead of changing the nozzles N to eject ink from the nozzle N3m in the front section in moving direction of the recording head H in the main scanning direction to the nozzle N3m+1 and to N3m+2 on the rear side, the driving phase is changed so that the nozzle N is changed from the nozzle N3m+2 on the rear side in the moving direction of the recording head H in the main scanning direction X to the nozzle N3m+1 and to N3m in the front section.
In doing so, as FIG. 27 shows, by increasing the moving speed of the recording head H in the main scanning direction X two times, the same nozzle N ejects ink at every other pixel. As FIG. 28 shows, by disposing another recording head H′ having nozzles M in the staggered arrangement in parallel to the recording head H in the main scanning direction X, or as FIG. 29 shows, by further disposing nozzles M in the staggered arrangement in parallel to the nozzles N in the main scanning direction X on the recording head H having the nozzles N in the staggered arrangement, and by driving the nozzles N as well as the nozzles M with an reverse phase for ejection, as FIG. 30 shows, the ink is ejected from the nozzles M and landed on gap sections to which the ink has been ejected from the nozzle N on the recording medium S so as to form the image on the recording medium S.
According to the above configuration, for example, even if some nozzles N fail and ink is not ejected from the nozzles N normally, the ink ejected from the normal nozzle M lands and fills a pixel position adjacent to the position on which the ink has not been ejected is supposed to land. Therefore, as FIG. 26 shows, it is prohibited that the portions when the ink is not ejected line up continuously in the main scanning direction X in the recorded image on the recording medium S, and a phenomenon that the streak-like pattern appears in the image can be prevented. Thus deterioration of image quality can be suppressed.
Meanwhile, in FIG. 27 (and in each figure below), the pulse width (time interval of high level of the pulse) of the strobe pulses STB1 to STB3 are described as if the pulse width is two times the pulse with of the strobe pulses STB1 to STB3 shown in FIG. 25. This is because the moving speed of the recording head D was increased two times and the pulse widths of the strobe pulses STB1 to STB3 themselves remain unchanged.